The United States Federal Government's NAVSTAR system, known generically as the global positioning system (GPS), provides worldwide positioning capability to its users with a system employing a set of fixed ground-based GPS controllers and a set of GPS satellites providing information suitable for use by passive GPS receivers. At any given time, there are at least 24 GPS satellites in operation, each orbiting Earth once every 12 hours at an altitude of 11,000 nautical miles. The position of each GPS satellite in the GPS system is calculated based on the relationship between that GPS satellite and one or more of the fixed ground-based GPS controllers.
Various components of the GPS system are operable to determine the distance between themselves, and therefore their respective positions, based on the time elapsed between the transmission of an electromagnetic signal by one GPS component and the receipt of the signal by another. Using this methodology, the GPS system has the capability to accurately determine the position of each GPS satellite with respect to the fixed ground-based GPS controllers, and therefore to the Earth itself.
Given that the electromagnetic GPS signals are traveling at the speed of light and that the distances involved are relatively short, the accuracy of the distance calculation depends on highly accurate timing synchronization, which is handled primarily with atomic clocks disposed within the various components of the system.
Each of the GPS satellites transmits signals to the other components of the GPS system. Civilian GPS satellite signals are transmitted at a frequency of 1575.42 MHz in the UHF band, while military GPS signals are transmitted at 1227.6 MHz. Signals at these frequencies can pass through clouds and fog, but will not pass through most solid objects such as buildings and mountains. Accordingly, a passive GPS receiver must have a clear line-of-sight to the GPS satellites necessary for positioning. A GPS satellite signal contains a pseudorandom satellite identification code, “ephemeris data” and “almanac data”. Ephemeris data reflects satellite status and current date and time. Almanac data discloses the position of the GPS satellite and other GPS satellites in the system.
Within this framework of GPS satellites having known positions at known times, a passive GPS receiver can determine its position with respect to the Earth using the signal delay reckoning method described above. Signals from multiple satellites are required in order to calculate the position of the passive GPS receiver. Given the signal from only a single GPS satellite, a passive GPS receiver can determine only that it is at a point on a sphere of a known radius centered on a GPS satellite having a known position. Given the signal from two GPS satellites, a passive GPS receiver can determine that it is at a point on the intersection of two spheres having known radii and known central points. Based on the principles of geometry, the intersection of two such spheres is a circle lying on the plane of intersection of the two spheres. Given the signal from three GPS satellites, a passive GPS receiver can determine that it is at a point on the intersection of three spheres having known radii and known central points. The intersection of three spheres is a set of two discrete points. Accordingly, given three GPS satellite signals, a passive GPS receiver can limit the range of its possible locations to two discrete points in three-dimensional space. In practice, it is often the case that only one of these two points is near the surface of the Earth. Given four or more GPS satellite signals, the location of the passive GPS receiver can be limited to a single discrete point within a certain margin of error. As the number of GPS satellites is increased, the margin of error is, of course, reduced.
The utility of a GPS receiver to the user is much improved through the inclusion of map display capability within the GPS receiver. With this capability, the user of a GPS receiver is able to reference his or her present global position to nearby roads, geographic landmarks, and other points of interest included in the map data stored within, and displayed by, the GPS receiver.